Motorblog

13 Motor Testing and CBM

In the early 1980’s, it was realized that voltage and current test results could be utilized in a different manner.  By using the electric motor, itself, as a transducer, the voltage and current waveforms could be evaluated by viewing the spectra resulting from an FFT (fast-fourier transform) of the sigantures.  Evaluation is performed by obtaining voltage and current data using voltage probes and current transformers digitized and evaluated.

VP, Electrical Reliability Group
T-Solutions, Inc.

MCSA and ESA

In the early 1980’s, it was realized that voltage and current test results could be utilized in a different manner.  By using the electric motor, itself, as a transducer, the voltage and current waveforms could be evaluated by viewing the spectra resulting from an FFT (fast-fourier transform) of the sigantures.  Evaluation is performed by obtaining voltage and current data using voltage probes and current transformers digitized and evaluated.

As current is the result of the action of the motor, most analyzers focused on the ‘Motor Current Signature,’ with the intention of viewing the condition of the motor airgap, rotor and some limited mechanical capabilities.  In the meantime, Howard Haynes, et.al., at Oak Ridge National Labs (ORNL) recognized that by comparing the voltage FFT and the current FFT, signatures could be sorted by those due to supply versus those due to the electric motor.  The ORNL system was originally developed and released in order to evaluate the condition of the rotor in Motor Operated Valves (MOV) in the nuclear power industry.  In particular, in areas where radiation contamination would not allow the operator to remove test equipment exposed to the radioactive environment of the containment area.  The concept was simple: How to electrically and mechanically evaluate the condition of equipment through a bulkhead.

The primary differences between MCSA and ESA became significant as time continued.  MCSA required a great deal of knowledge about the system being reviewed and potential signatures had to be determined in advance, as distortions in voltage would impact the current signature.  ESA allowed the ability to compare the voltage signature and current signature, this provided the ability to, at a glance, determine if a signature was the result of the supply power or load within a single dataset.  Additionally, ESA provided the ability to troubleshoot upstream (ie: control) problems and downstream (ie: motor and load) problems in a single snapshot.

Basic Principles

For this part, we will start with theory, and a ‘perfect’ motor.  Within this perfect motor, the windings are balanced, the rotor bars are good and the airgap is concentric.  A perfect sinusoidal voltage is applied to the electric motor.  As the motor runs, the transformer ratio between the stator (primary) and rotor (secondary) remains balanced.  The combined stator and rotor load generates a current waveform that lags behind the voltage waveform, slightly.  The result is a perfect sinusoidal signal being injected into the motor and a perfect sinusoidal signal providing feedback from the motor.

Now, assuming that the voltage remains ‘perfect,’ we now introduce an eccentric rotor condition.  The rotor is now closer to one or more coils (and phases) and further from others.  Due to the inverse square law for fields, the strength of the magnetic field, across the airgap, closest to the rotor increases by the square of the distance, while the opposite side diminishes by the square of the distance.  The resulting output current is now no longer sinusoidal as the airgap fields expand and collapse.

If we then introduce a variation into the voltage, this signature is introduced into the airgap magnetic fields of the motor and will result in a similar signature showing in the current, but at a slightly lower value.  The differences in magnitude, in order to determine whether the signature is coming from voltage or current, can be determined by viewing the results in dB.

Demodulation

Because the magnitude of voltage and current can hide some subtle problems, ie: early bearing faults, etc., a process of demodulation is introduced.  This process involves stripping the actual sine-wave, which acts as the carrier signal for what we are actually looking for, and leaving the remaining signature.  That is basically it, although the algorithms can be quite complicated.

To be continued…